Image forming apparatuses, which form images using an intermediate transfer method including forming a toner image on an image bearing member such as photoreceptors, primarily transferring the toner image onto an intermediate transfer medium, and secondarily transferring the toner image onto a recording sheet, are well known. By using an elastic material having good flexibility for the intermediate transfer medium, the adhesion between the recording sheet and the toner image can be enhanced.
Image forming apparatuses using such an intermediate transfer method typically include four photoreceptors which serve as image bearing members and on which yellow (Y), magenta (M), cyan (C), and black (K) toner images are respectively formed, and an endless intermediate transfer belt which makes endless movement while being contacted with the four photoreceptors to form primary transfer nips at which the Y, M, C and K toner images are sequentially transferred from the photoreceptors to the intermediate transfer belt so that the color toner images are overlaid, resulting in formation of a combined color toner image on the intermediate transfer belt. The combined color toner image on the intermediate transfer belt is then secondarily transferred onto a recording sheet, which is timely fed to a secondary transfer nip formed by the intermediate transfer belt and a transfer roller.
In such image forming apparatuses, since the amount (weight) of toners constituting the combined color toner image is considerably large, the combined color toner image cannot be satisfactorily transferred onto a recording sheet (resulting in formation of an image having a low image density) unless a large secondary transfer current is applied to the secondary transfer nip. In this regard, if a larger amount of secondary transfer current than that needed for transferring a toner image is applied, discharge is caused between the recording sheet and the intermediate transfer belt at the secondary transfer nip, thereby charging the toner of the toner image so as to have a charge with the opposite polarity, resulting in occurrence of defective transferring (i.e., formation of a low density image).
In attempting to solve the problem, a technique such that a proper amount of transfer current is determined based on the ratio of the area of a first toner image to the area of the overlapped portion of the first toner image and a second toner image, and the thus determined proper transfer current is applied to a transfer blade to satisfactorily transfer the combined toner image is proposed.
However, even when such a technique is used, a defective transfer problem such that a specific color portion of a combined toner image is not satisfactorily transferred is caused depending on the configuration of the combined toner image (such as area ratios of the color toner images constituting the combined toner image).
For these reasons, the present inventor recognized that there is a need for an image forming apparatus which can produce high quality multicolor images without causing the defective transfer problem even when the area ratios of multiple color toner images change.